Controlled synthesis and analysis of He-H⁺ <inf>3</inf> in a 3.7 K ion trap

Abstract

© 2015 Taylor & Francis. Complexes of the triatomic hydrogen ion with helium were synthesised in a low-temperature 22-pole rf ion trap at He number densities of up to 10¹⁶ cm^-³. Absolute ternary rate coefficients for sequentially attaching He atoms have been determined from the growth of complexes with increasing storage time. The number of helium-tagged ions is significantly reduced when increasing the nominal temperature from 4 to 25 K. Competition between attachment and dissociation via collisions leads to stationary H<inf>e</inf><inf>n</inf>-H⁺<inf>3</inf> (n up to 9) distributions. State-specific excitation of the trapped H⁺<inf>3</inf> ions via IR transitions significantly reduces the formation of complexes. Tuning the laser to Δv<inf>2</inf> = 1 transitions in the range of 2726 cm^-¹ leads to LIICG lines, i.e., to spectra caused by laser-induced inhibition of complex growth. In addition, almost 100 lines have been found between 2700 and 2765 cm^-¹, which are attributed to laser-induced dissociation of the in situ formed He-H⁺<inf>3</inf> complex ions. These lines are not yet assigned; however, their absorption strength, statistics and predissociation lifetimes provide interesting information on both the stable complexes as well as on scattering resonances in low-energy H⁺<inf>3</inf>+He collisions. New calculations of the potential energy surface will help to analyse the dissociation spectrum. There are some indications that para-H⁺<inf>3</inf> is enriched under the conditions of the present experiment.